Automatic transmissions have long been employed in automotive vehicles for transmission of engine torque to the drive wheels of the vehicle. These transmissions are controlled to execute shifting between several gear ratios depending on engine speed and operator commanded vehicle speed. Automatic transmissions have used a hydraulic fluid for transmission of torque between rotating driving and driven members of the device. This automatic transmission fluid (ATF) is subjected to considerable shear forces in the operation of the torque converter and transmission.
Automatic transmission fluids typically comprise a base oil with additives to slow thermal degradation of the oil. An ATF is heated due to the energy that is put into it during operation of the torque converter and transmission as well as heat from the engine compartment environment. The fluid may experience temperatures of 160° C. or higher. Over the past decades, vehicle manufacturers have recommended ATF change intervals of 50,000 miles, 100,000 miles, or fill for life depending on severe, normal or mild operating conditions, respectively.
Now the trend in vehicle requirements is for smaller sump transmissions, more aggressive shift calibrations and lower cooling capacity. These requirements mean that an ATF may experience a more severe operating environment, resulting in faster degradation due to oxidation and high shift energy input. These more severe demands on the fluid may require shorter, less predictable change periods. Accordingly, it is an object of this invention to provide a method for on-vehicle computer execution for predicting the end of the useful life of a vehicle's ATF and for advising the vehicle operator to change the fluid.